1. Field of the Invention
This invention relates to a viscosity index improver (hereinafter referred to as VII or VI improver). More particularly, it relates to a VII for lube oils, such as automatic transmission fluids (ATF), gear oils, hydraulic fluids and engine oils.
2. Description of the Prior Art
As VI improvers to improve viscosity index (hereinafter referred to as VI) of lube oils, there have been known heretofore copolymers of C1-18alkyl (meth)acrylates (for instance, U.S. Pat. No. 5,622,924).
The term xe2x80x9c(meth)acrylatexe2x80x9d represents acrylate and/or methacrylate and also xe2x80x9cC1-18alkylxe2x80x9d represents alkyl group containing 1-18 carbon atoms; and similar expressions are used hereinafter.
It is an object of the present invention to provide a VII capable of imparting an improved VI to a lube oil.
It is another object of this invention to provide a VII capable of imparting an improved low temperature viscosity characteristics to a lube oil.
It is still another object of the invention to provide a VII having an improved shear stability.
It is yet another object of the present invention to provide a high VI lube oil having an improved shear stability.
It is further object of the present invention to provide an automotive lube oil capable of meeting fuel cost-saving requirement of an automobile.
It is still further object of the present invention to provide a monomer suitable for producing a VII.
Briefly, these and other objects of this invention as hereinafter will become more readily apparent have been attained broadly by a VII, comprising a polymer (A1) or (A0) as follows.
Polymer (A1) has a solubility parameter (hereinafter referred to as SP) of 8.6-9.4, a crystallizing initiation temperature (hereinafter referred to as Tc) of at most xe2x88x9215xc2x0 C. and a steric hindrance factor F, defined by the following equation (1), within the range of 0-13.
F=4X+Yxe2x80x83xe2x80x83(1) 
In the equation (1), X and Y represent the total atom numbers at the 6th position and the 7th position, respectively, in the side chain(s), counted from the backbone.
Polymer (A1) comprises units of at least one monomer selected from the group consisting of the following monomers (a)-(d):
(a) C1-40alkyl group-containing (meth)acrylates,
(b) alkyl alkenyl ethers,
(c) alkenyl carboxylates, and
(d) nitrogen-containing unsaturated monomers.
Polymer (A0), used in another aspect of the invention, comprises units of at least one branched C20-40alkyl group-containing (meth)acrylate (a0) and units of at least one other alkyl (meth)acrylate. Said (meth)acrylate (a0) is capable of providing a homopolymer having a Tc of at most 5xc2x0 C.
Polymers (A1) and (A0), hereinafter generically referred to as Polymers (A), have a weight-average molecular weight (hereinafter referred to as Mw) of usually 5,000-2,000,000.